At Brutus's house, Portia, nearly hysterical, orders Lucius to run to the Capitol. She wants news of the conspirators' success or failure, but cannot think what to tell Lucius, who mustn't know of Brutus's plans. Portia, aside, bemoans the female weakness that makes it so hard for her to keep secrets.
Portia's panic here contrasts her earlier strength. She sees her weakness as being characteristic of her gender, just as her fortitude was uncharacteristic.
The Soothsayer passes, and Portia asks if he is going to the Capitol. He says that he is, to warn Caesar. Portia asks if he knows of any specific threat against Caesar; he says he doesn't know anything specific, only fears. The Soothsayer continues on. Portia sends Lucius after Brutus with only a greeting, and then goes inside.
The fact that the Soothsayer himself attempts twice to warn Caesar would seem to indicate that his assassination is a possible outcome, not an inevitable one—if it is inevitable, why bother warning him?